"I really don't think they should let the other sort in, do you? They're just not the same, they've never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, imagine. I think they should keep it in the old wizarding families."

Pure-blood supremacy[1] is the belief that wizards and witches whose known ancestry is exclusively magical (pure-bloods) are inherently superior to wizards and witches who have traceable Muggle ancestry (Muggle-borns and half-bloods). Proponents of this ideology typically regard Muggle-born wizards as impure and not worthy of possessing magical ability and often actively discriminate against them.

Pure-blood supremacists consider themselves to be the elite of the wizarding world, akin to royalty. They look down upon half-bloods, Muggle-borns, and half-breeds, and consider the Muggle world to be inferior to their own. Pure-bloods, and even half-bloods, who do not share this view are considered traitors to their own kind, hence the term "blood traitor." They are considered disgraces by pure-blood supremecists such as the Malfoys.

Elitist pure-bloods believe that it is a sign of weak magic to enjoy non-magical company[2] and seem to believe that some of the supposed dirtiness of Muggles and Muggle-borns will rub off on those who associate with them, thus they consider blood traitors to be "filthy" as well[3].

Some extremists consider blood traitors to be nearly as bad as the Muggle-borns they are prejudiced against. Bellatrix Lestrange, for example, claimed that "blood traitor is next to Mudblood in [her] book,"[4] and her family often disowned and blasted off the family tree any members who could be considered blood traitors. Blood traitors might also be shunned by their families and acquaintances; for example, Bellatrix and her younger sister Narcissa Malfoy stopped seeing their sister Andromeda after she married a Muggle-born wizard, Ted Tonks.[4]

Degrees of prejudice

Pure-blood elitism

The most extreme of the pure-blood supremacists seek to purge the world of those they consider "impure." They often tolerate half-bloods out of necessity, but muggle-borns are persecuted, and muggles are viewed as nothing more than animals.

The Blacks were an infamous pure-blood family who believed that being pure-bloods made them almost royalty, and disowned "blood traitor" family members. Many members of the House of Black were disowned for marrying muggles or muggle-borns, or for refusing to uphold the family's prejudiced views. Even being a Squib is considered to be impure, and will be disowned.

Despite the fact that he lived in poverty, Gaunt was obsessed with being pure-blood and certain of his superiority over those of "lesser" blood status. He disowned his daughter, Merope, for her little magical ability and her blood treachery.

Possibly the biggest muggle-haters and blood-purity fanatics of their time. Led by Voldemort, who thought that magical blood was pure and special. Under their regime, muggle-borns were stripped of their wands and reduced to beggars, and muggles were killed indiscriminately.

Slytherin did not accept muggle-borns in his house and subsequently ended his friendship with Godric Gryffindor. He left a Basilisk in a hidden chamber beneath Hogwarts so that it could one day purge the school of muggle-borns.

Brutus Malfoy was a campaigner of purity. He wrote the paper, Warlock at War which promoted wizarding purity. He hated Muggles and blood traitors and seemingly became famous with pure-blood elitist communities. After Muggles persecuted wizards, he began to grow a pure-blood supremacist ideology.

Pure-blood bias

"Your mother was Muggle-born, of course. Couldn't believe it when I found out. Thought she must have been pure-blood, she was so good."

Although Slughorn thought that pure-bloods were normally more magically skilled, he nonetheless had many talented muggle-borns in his Slug Club, including Lily Evans and Hermione Granger. He even wanted Lily Evans to be in his house of Slytherin.

Fudge showed great favouritism toward influential pure-bloods such as the Malfoys. However, until 1995, he relied heavily on the counsel of Albus Dumbledore, a supporter of muggle-rights and was very polite to Harry Potter, a half-blood. He also had a cordial, if somewhat patronising, relationship with the muggle Prime Minister and was highly distraught at the gruesome murder of twelve muggles earlier in his days at the Ministry. However, Fudge allowed the passage of anti-Half-breed legislation.

During his time at Hogwarts, Severus fell in with a group of future Death Eaters in his desire for power and acceptance.[6] Despite his childhood claim that blood status didn't matter[4], he began to use the term "Mudblood" to describe muggle-borns. However, an accidental use of the word to Lily Evans ended their friendship. He later regretted this, and no longer tolerated the use of the word or any Pure-blood bias in his presence.

Wizarding supremacists

"Gellert – Your point about Wizard dominance FOR THE MUGGLES' OWN GOOD – this, I think, is the crucial point. Yes, we have been given power and yes, that power gives us the right to rule, but it also gives us responsibilities over the ruled."

—A letter from the young Albus Dumbledore to Gellert Grindelwald.[src]

Many wizards, even those who oppose discrimination based on blood status, regard themselves as superior to the non-magical muggles. While many are content with simply remaining separate from muggle society, some believe that the world would be a better place with wizards in charge.

As a young man, Dumbledore resented the damage a group of muggle boys had done to his sister Ariana, and the impact it had on his family. With his friend Grindelwald, he dreamed of establishing wizard dominance in the world for the muggles' own good. After Ariana's death, he came to regret his views, and spent the rest of his life as a fierce advocate of equality and tolerance.

With his friend Albus, Grindelwald planned to bring the wizarding world out of hiding, bringing muggles under wizarding control and guidance "for the greater good".

Percival Dumbledore gained a reputation as a muggle-hater for his attack on three muggle boys. What was not widely known however, was that his attack was an act of retribution for their assault on his daughter Ariana, which left her severely traumatised. He never revealed the true motives for his attack to prevent his daughter from being confined to St. Mungo's for the rest of her life. As his wife Kendra was a muggle-born witch, it is debatable whether Percival harboured any prejudicial views.

The Wizard in The Wizard and the Hopping Pot believed that Muggles were worthless, in contrast with his father's attitude. However, after the Hopping Pot began relentlessly tormenting him for not helping his Muggle neighbours, he changed his opinion and started helping the Muggles.

↑Neville Longbottom says of the Carrows in Chapter 29 of Deathly Hallows: "They don’t want to spill too much pure blood, so they’ll torture us a bit if we’re mouthy but they won’t actually kill us.” Given this statement, and the fact that the Weasleys were only in direct danger after Ron Weasley's help of Harry Potter was revealed to the Death Eaters, it seems that they were hesitant to kill blood traitors unless they were very rebellious.